Multiwaveband polarimetric observations of NRAO 530 on parsec‐scale

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 10/2010; 408(2):841 - 848. DOI: 10.1111/j.1365-2966.2010.17193.x
Source: arXiv

ABSTRACT We report on Very Long Baseline Array polarimetric observations of NRAO 530 at 5, 8, 15, 22 and 43 GHz made during one week in 1997 February. We present the total intensity, the fractional polarization and the electric vector position angle (EVPA) distributions at all these frequencies. A model fitting has been performed to the full polarization visibility data. From this, the fitted southernmost component A is confirmed as the core of the radio structure with relatively high brightness temperature and hard spectrum between 15 and 43 GHz in comparison with the central component B of dominant flux. The relatively high degree of polarization for the component A may arise from its complex radio structure, which can be resolved at 86 GHz. In contrast, the component B shows a well-fitted power-law spectrum with a spectral index of about −0.5 (f∝να), and a linear correlation between EVPAs and wavelength square with an observed rotation measure of about −1062 rad m−2, indicating its structural singleness. Assuming that the component B has a comparable degree of polarization without depolarization at these frequencies, the decrease in fractional polarization with wavelength mainly results from opacity and Faraday rotation, in which the opacity plays quite a large role. A spine-sheath-like structure in fractional polarization (m) is detected, covering almost the whole emission region at 5 and 8 GHz, with a degree of polarization relatively low along the jet spine, becoming higher towards two sides of the jet. The linear polarization at 5 GHz shows three separate polarized emission regions with alternately aligned and orthogonal polarization vectors down the jet. The polarization goes to zero between the top two regions, with the highest polarization level occurring at the top and bottom. The 5- and 8-GHz images show EVPA changes across the width of the jet as well as along the jet. These complex polarimetric properties can be explained in terms of either the presence of a large helical magnetic field or tangled magnetic fields compressed and sheared down the jet. These can be further determined by multifrequency polarimetric very long baseline interferometry observations with sufficient high resolution and sensitivity spanning an appropriate frequency range.

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    ABSTRACT: NRAO 530 is an optically violent variable source and has been studied with multi-epoch multi-frequency high-resolution VLBI observations. NRAO 530 was monitored with the VLBA at three frequencies (22, 43 and 86 GHz) on 10 consecutive days in 2007 May during observations of the Galactic Center (Sgr A*). Furthermore, analysis of archival data of NRAO 530 at 15 GHz over the last ten years allows us to study its detailed jet kinematics. We identified the compact component located at the southern-end of the jet as the VLBI core, consistent with previous studies. The 10-d monitoring data at the 3 high frequencies were shown to produce high quality and self-consistent measurements of the component positions, from which we detected for the first time a two-dimensional frequency-dependent position shift. In addition, the repeated measurements also permit us to investigate the interday flux density and structure variability of NRAO 530. We find that it is more variable for the inner jet components than those further out. We obtained apparent velocities for eight jet components with $\beta_{\rm app} ranging from 2 to 26 c. Accordingly, we estimated physical jet parameters with the minimum Lorentz factor of 14 and Doppler factors in the range of 14--28 (component f). The changes in the morphology of NRAO 530 were related to the motion of separate jet components with the most pronounced changes occurring in the regions close to the core. For NRAO 530, we estimated a P.A. swing of $3^{\circ}.4$ per year for the entire inner jet (components d and e). The non-ballistic motion and change of jet orientation makes this source another prominent example of a helical and possibly `swinging' jet.
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    ABSTRACT: We report on VLA observations of HST-1 in M87 at 8 GHz from 2003-2007, during which a long major outburst occurs from radio to X-ray wave bands. At the VLA resolution, the flux density of HST-1 rises rapidly from 2003, peaks at the end of 2004, and then falls slowly in subsequent stages, which is similar to that in optical and X-ray wave bands. It appears that HST-1 moves with an apparent speed of 1.23 c±0.91 c, and the fractional polarization keeps rising through the whole major outburst. The persistent increase in polarization level may mainly be attributed to the formation of a couple of new `subcomponents' of relatively high degree of polarization within HST-1, and the weakening depolarization due to Faraday rotation and/or opacity through the whole major outburst.
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